System and method for diagnosing an exhaust gas sensor

Abstract

A method and system for monitoring an exhaust gas sensor coupled in an engine exhaust is provided. In one example, the method determines an estimate of an exhaust gas oxygen sensor time constant according to a comparison of air/fuel ratios and a system time constant.

The invention claimed is: 
     
       1. A method of monitoring an exhaust gas sensor in an engine exhaust passage, comprising:
 generating an indication of exhaust gas sensor degradation via a controller according to a time constant of the exhaust gas sensor exceeding a threshold during a fuel cut-off entry and exit, the time constant of the exhaust gas sensor estimated based on a comparison of air/fuel ratios of the exhaust gas sensor; and 
 adjusting engine fuel delivery based on the time constant. 
 
     
     
       2. The method of  claim 1 , where the time constant is an amount of time that it takes for the output of the exhaust gas sensor to reach a predetermined percentage of a final output of the exhaust gas sensor responding to a step change of exhaust gas constituents. 
     
     
       3. The method of  claim 2 , wherein the step change of exhaust gas constituents is generated via performing a step change in an engine air-fuel ratio. 
     
     
       4. The method of  claim 1 , wherein generating the indication of exhaust gas sensor degradation includes displaying a message via a human/machine interface. 
     
     
       5. The method of  claim 1 , further comprising adjusting engine fuel control in response to the indication of exhaust gas sensor degradation. 
     
     
       6. The method of  claim 1 , further comprising transitioning from combusting a first air-fuel ratio mixture in an engine to which the engine exhaust passage is coupled to combusting a second air-fuel ratio mixture in the engine to stimulate the output of the exhaust gas sensor during the time period, where the first air-fuel ratio mixture is leaner than the second air-fuel ratio mixture. 
     
     
       7. The method of  claim 1 , further comprising transitioning from combusting a first air-fuel ratio mixture in an engine to which the engine exhaust passage is coupled to combusting a second air-fuel ratio mixture in the engine to stimulate the output of the exhaust gas sensor during the time period, where the first air-fuel ratio mixture is richer than the second air-fuel ratio mixture. 
     
     
       8. A system for a vehicle, comprising:
 an engine including a fuel injection system; 
 an exhaust gas sensor coupled in an exhaust system of the engine; and 
 a controller including instructions executable to:
 estimate an oxygen sensor time constant via monitoring a current air/fuel ratio and a previous air/fuel ratio during a fuel cut-off entry and exit; 
 comparing the current and previous air/fuel ratios to a starting air/fuel ratio and an expected final air/fuel ratio; 
 indicate a presence or an absence of degradation of the exhaust gas sensor in response to the oxygen sensor time constant; and 
 adjust engine fuel delivery based on the oxygen sensor time constant. 
 
 
     
     
       9. The system of  claim 8 , wherein the previous air/fuel ratio is directly before the current air/fuel ratio. 
     
     
       10. The system of  claim 9 , wherein the oxygen sensor time constant is estimated via a reverse half rule approximation. 
     
     
       11. The system of  claim 8 , wherein the time constant is further based on a calibrated value specific to a cylinder bank, the fuel cut-off entry, and the fuel cut-off exit. 
     
     
       12. The system of  claim 8 , further comprising additional instructions to transition from combusting a first air-fuel ratio mixture in the engine to combusting a second air-fuel ratio mixture in the engine to stimulate an output of the exhaust gas sensor during a time period in which the rate of change of the signal is determined, where the first air-fuel ratio mixture is leaner than the second air-fuel ratio mixture, and wherein the transition includes exiting a fuel cut-off event. 
     
     
       13. The system of  claim 8 , further comprising additional instructions to transition from combusting a first air-fuel ratio mixture in the engine to combusting a second air-fuel ratio mixture in the engine to stimulate an output of the exhaust gas sensor during a time period in which the rate of change of the signal is determined, where the first air-fuel ratio mixture is richer than the second air-fuel ratio mixture, and wherein the transition includes entering a fuel cut-off event. 
     
     
       14. The system of  claim 8 , wherein the presence or absence is indicated via a human/machine interface. 
     
     
       15. A method of monitoring an exhaust gas sensor in an engine exhaust passage, comprising:
 generating an indication of exhaust gas sensor degradation via a controller according to a time constant of the exhaust gas sensor exceeding a threshold, the time constant of the exhaust gas sensor estimated based on based on a comparison of air/fuel ratios of the exhaust gas sensor during a fuel cut-off entry and a fuel cut-off exit; and 
 adjusting parameters of a fuel controller according to the time constant of the exhaust gas sensor to adjust engine fuel delivery based on the time constant. 
 
     
     
       16. The method of  claim 15 , wherein the air/fuel ratios included in the comparison comprise a current air/fuel ratio and a previous air/fuel ratio compared to a starting air/fuel ratio and an expected final air/fuel ratio. 
     
     
       17. The method of  claim 16 , wherein the previous air/fuel ratio is directly before the current air/fuel ratio. 
     
     
       18. The method of  claim 15 , wherein the monitoring occurs during a fuel cut-off event in response to inputs provided by a vehicle operator via an accelerator pedal and/or a brake pedal. 
     
     
       19. The method of  claim 18 , wherein the time constant is further based on a calibrated value, the calibrated value calibrated to one or more of a cylinder bank, entry conditions to the fuel cut-off event, and exit conditions to the fuel cut-off event.